Optical masbr comprising the active medium capa



June 28, 1966 L. F. JOHNSON 3,258,715

OPTICAL MASER COMPRISING THE ACTIVE MEDIUM caF tNd Filed NOV. 20, 1961 H1 F A jh 6 INVENTOR. L.E'JOHNSON MM LT-N 'AQ A T TORNE V United StatesPatent 3,258,715 OPTICAL MASER COMPRISING THE ACTIVE MEDIUM CaF :Nd

Leo F. Johnson, North Plainfield, N.J., assignor to Bell TelephoneLaboratories, Incorporated, New York,

N.Y,, a corporation of New York Filed Nov. 20, 1961, Ser. No. 153,603 4Claims. (Cl. 331-945) This invention relates to optical masers for usein the infrared portion of the optical spectrum. The materials of theinvention include a calcium fluoride host lattice and additionallycontain neodymium ions in the 3+ valence state.

Recently, considerable attention has been focused on a new class ofsolid state maser devices which are capable of generating or amplifyingcoherent electromagnetic wave energy in the optical frequency range.Devices of this type, which are described, for example, in the UnitedStates Patent 2,929,922 to Schawlow and Townes, are considered to beoperable over the spectral range from far infrared to ultraviolet, anequivalent bandwidth of about cycles. Such a bandwidth is capableof'providing a great number of new communications channels, there bymultiplying the number of available channels which has heretofore beenlimited by the characteristics of the heavily used lower frequencyportions of the spectrum.

Typically, a maser of the type known in the art employs an activematerial characterized by a plurality of distinct energy levels, theseparation of these levels corresponding to frequencies within thedesired operating frequency ranges. More particularly, the separationbetween two of the energy levels E and E corresponds to quantized waveenergy having a frequency v given by Bohrs equation where h is Plancksconstant.

In accordance with the maser principle, wave energy in an appropriatefrequency range is applied to an ensernble of paramagnetic ions, therebypumping electrons from a lower energy level to a metastable higherlevel. The excited electrons tend to remain in the upper level for ashort time before decaying or relaxing to the lower level. The downwardtransition is normally accompanied by the radiation of wave energy ofthe frequency corresponding to separation between the energy levelsconcerned. During maser operation sufficient pump power is supplied tothe active medium to produce, at least intermittently, a nonequilibriumpopulation distribution beween the pair of energy levels. Moreparticularly, the population of the upper level is increased withrespect to that of the lower level. When the population of the upperlevel exceeds that of the lower level, a population inversion ornegative temperature is considered to obtain. Operation of the maser isdependent upon the fact that a small signal at the proper frequency actsto stimulate the downward transition of the excited electrons from themetastable state, and that the stimulated emission is coherent and inphase with the signal.

Among the more promising forms of maser are those which utilize anactive medium characterized by first, second, and third successivelyhigher electron energy levels. Typically, in optical maser materials theupper level is relatively broad, and in some cases is best described asa band. Continuous wave operation of such three-level devices may beachieved, for example, by

pumping electrons from the first to the third level fromwhich they relaxspontaneously, by nonradiative proc- Patented June 23, 1966 esses, tothe second level, thereby producing the desired population inversionbetween the second and first levels. Advantageously, in masers of thistype, the relaxation time between the third and second levels is shorterthan that between the second and first levels so that the population ofthe second level may be continuously maintained during operation of thedevice. Additionally, as the magnitude of the negative temperatureattained depends on the relative populations of the first and secondlevels, the energy level system of preferred materials also includemechanisms which continuously depopulate the terminal state of theoptical transition. Thus, the population inversion is maintained at arelatively high value and maser action is facilitated.

Among the more promising active maser materials are those which comprisea host crystal containing paramagnetic ions from which the stimulatedemission occurs. The host crystal must be of a material capable ofaccepting the paramagnetic ions in such a way that they are able, uponexcitation, to fluoresce with good over-all quantum efliciency, with asmuch as possible of the emit ed energy concentrated in a single narrowspectral line. More particularly, the host must accept the ions in sucha way as to minimize coupling between them and the crystal lattice atthe maser frequency, while simultaneously permitting relaxation from thepump band to the metastable state. Additionally, the host crystal shouldbe of good optical quality. That is to say, it must be relatively freeof scattering centers and hence transparent to light waves at theoperating frequency of the maser. Furthermore, the host should have alow coefficient of absorption at the pump frequency-to mimimize heatingof the maser medium and to promote more eflicient utilization of thepump power. Chemical and physical stability are further desiderata. Itis also required that the host be a mechanically workable material,capable of being accurately shaped and highly polished.

A combination of paramagnetic ions with a host lattice meeting the abovementioned conditions is ruby, which continues to be widely used as anoptical maser medium and is, in fact, one of the very few materialswhich have been operated successfully. Ruby has usable emission lines at.6943 micron and .6921 micron. In addition, ruby having a highconcentration of chromium ions is characterized by sharp satellite linesat .7009 micron and .7041 micron. Another successful material, operableat liquid hydrogen temperatures, comprises a calcium fluoride hostlattice containing samarium ions. Emission from the samarium ions is at.7082 micron. A more recently discovered optical maser medium, disclosedin copending application Serial No. 139,266, filed September 19, 1961,of Johnson and Nassau, is calcium tungstate containing trivalentneodymium ions. Additional newly discovered materials are disclosed incopending applications Serial Nos. 153,605, 153,606, 153,607 and153,604, all filed concurrently and assigned to the assignee hereof.These include praseodymium in calcium tungstate operating at about 1.047microns; neodymium in strontium molybdate operating at about 1.06mircons; holmium in calcium tungstate operating at about 2.046 microns;and thulium in calcium tungstate operating at about 1.91 microns.

Ruby optical masers of the usual design are subject to the disadvantageof requiring a high pump power to establish the required populationinversion. The amount of power supplied to the ruby and the conditionsof its absorption by the crystal have thus far limited ruby maseroperation to producing a pulsed beam of coherent light. It is apparent,however, that in many applications 3 it is highly desirable to producecontinuous coherent light beams by maser action.

Furthermore, it is to be noted that the choice of active medium for anoptical maser device governs the frequency of the usuable emissionlines. Thus, it is desirable to provide a variety of optical masermaterials in order to make possible the generation and amplification ofcoherent light beams over the wide range of the optical frequencyspectrum. However, despite the discovery of several operable materials,on the basis of present inforration there is no known theory which isable to predict which combinations of ions and host lattices will besuccessful.

An object of the invention is the generation and amplification ofcoherent radiation in the infrared portion of the optical frequencyspectrum.

It is also an object of the invention to provide an optical masercapable of operation in the infrared range and requiring a relativelylow pumping power.

In accordance with this invention, a new fluorescent material suitablefor use in optical masers has been discovered. This material consists ofa calcium fluoride host lattice in which some of the calcium ions havebeen replaced by neodymium ions in the 3+ valence state. Trivalentneodymium is the active maser medium and stimulated emission isproduced. At 77 degrees K., for example, stimulated emission is producedat 1.046 microns.

A feature of the invention is an optical maser having an active mediumconsisting of trivalent neodymium ions in a calcium fluoride hostcrystal.

An optical maser in accordance with the invention is shown in thefigure. There is depicted a rod-shaped crystal 1 of calcium fluoridehaving an appropriate small concentration of trivalent neodymium ions asdisclosed herein. Pump energy is supplied by a helical lamp 2encompassing rod 1 and connected to an energy source not shown. Ends 3and 4 of rod 1 are ground and polished in the form.of confocal sphericalsurfaces. Reflective layers 5 and 6 are deposited on ends 3 and 4,thereby forming an optical cavity resonator of the type described in acopending patent application Serial No. 61,205, filed October 7, 1960,by Boyd, Fox and Li. Advantageously, layer 6 is highly reflecting whilelayer 5 includes at least a portion which is only partially reflectingto permit the escape of coherent radiation 7 having a wavelength ofabout 1.046 microns. If desired, rod 1 during operation may bemaintained in a bath of a liquified gas, such as nitrogen, to maintain alow temperature with the desirable results hereinafter described.

The lamp 2 is advantageously of'a type which produces intense radiationover a broad band extending from about 0.3 micron to longer wavelengths.Mercury or Xenon lamps are considered useful to pump the material of theinvention, which is characterized by a plurality of very sharpabsorption lines in the specified spectral range. Other types of lampsmay, of course, be employed provided they emit sufficient energy atwavelengths corresponding to one or more useful absorption lines of thematerial. Electrons in the active medium are excited to upper energylevels by the pump power and relax through nonradiative processes to anintermediate level corresponding to the level of free neodymium ions.This level corresponds to the metastable level of the above-mentionedexemplary three-level system. A negative temperature is thus createdbetween the 1 and the *I levels. The 1 level lies about 2,000 cm. abovethe ground state and has a negligible population at room temperature.The population of the terminal state may be further reduced by coolingthe crystal. Stimulated emission at 1.046 microns in the infrared isproduced by this device.

An optical maser of the type illustrated in the figure has been operatedusing as an active medium calcium fluoride containing 0.1 atomic percentof neodymium in place of calcium. The device produced intense emissionat 1.046 microns in the infrared. Maser action may be achieved over arange off neodymium concentration. Although in principle there is nolower limit on the concentration of Nd+ which may be employed in thecrystal 1, yet a practical limit of about 0.01 percent is imposed by thenecessity of having suificient unpaired electrons available to produce auseful output.

Additionally, the preferred concentration of Nd ions in the CaF hostcrystal is deemed to be about 1 percent. Such a concentration appearsdesirable from the standpoint of maximum intensity in the narrowestpossible line. However, useful concentrations are deemed to extend asfar a 5 percent Nd+ Beyond 5 percent increasing account must be taken ofline broadening due to interaction among the neodymium ions themselves.Although stimulated emission may be obtained with concentrations as highas 5 percent, most applications will require a maximum of about 3percent. Furthermore, crystals having more than about 5 percentneodymium ions are somewhat more difiicult to manufacture than are thosehaving smaller concentrations.

From measurements made on the device described above, it is estimatedthat, at liquid nitrogen, the power required to operate a maser inaccordance with the invention is about 5 percent of that required tooperate a ruby maser of similar configuration under the same condition.Active maser media based on Samarium, for example, while operable atliquid hydrogen temperature, are not operable at liquid nitrogentemperature.

Although the invention has been described with reference to a specificembodiment, this is to be construed by way of illustration and does notlimit the scope of the invention. For example, the material of theinvention may be used with any concentration of neodymium in the rangesset forth. Furthermore, the material may be used in optical cavityresonators other than the confocal type. The parallel plate resonator,as well as others, may also be employed. Other variations are alsopossible within the spirit of the invention.

What is claimed is:

1. An optical maser comprising an active medium consisting essentiallyof a substantially monocrystalline calcium fluoride host lattice inwhich a portion of the calcium ions have been replaced by neodymium ionsin the trivalent state, the portion of calcium ions so replaced being inthe range of from 0.01 percent to 5 percent, means for producing apopulation inversion between a pair of optically connected energy levelsof said neodymium ions, and means for stimulating coherent emission atthe wavelength corresponding to the energy separation of said levels.

2. An optical maser comprising an active medium consisting essentiallyof a substantially monocrystalline calcium fluoride host lattice inwhich a portion of the calcium ions have been replaced by neodymium ionsin the trivalent state, the portion of calcium ions so replaced being inthe range of from 0.01 percent to 3 percent, means for producing apopulation inversion between a pair of optically connected energy levelsof said neodymium ions, and means for stimulating coherent emission atthe wavelength corresponding to the energy separation of said levels.

3. An optical maser comprising means forming an optical cavityresonator, a negative temperature medium disposed within said resonator,said medium consisting essentially of a substantially monocrystallinecalcium fluoride host lattice in which a portion of the calcium ionshave been replaced by neodymium ions in the trivalent state, means forpumping said medium to produce a population inversion therein between apair of optically connected energy levels of said neodymium ions, andmeans for abstracting from said cavity resonator c0- herent light waveenergy at the Wavelength corresponding to the energy separation of saidlevels.

4. An optical maser comprising a rod shaped substantiallym-onocrystalline negative temperature medium consisting essentially ofcalcium fluoride in which from .01 to 5 percent of the calcium ions havebeen replaced by trivalent neodymium ions, sa-id rod having confocalspherical end surfaces, said end surfaces being covered with areflective layer thereby forming an optical cavity resonator in saidmedium, pumping means disposed about 10 said rod for producing therein apopulation inversion between a pair of optically connected energy levelsof said neodymium ions, and means for abstracting from said cavitycoherent emission of the wavelength corresponding to the energyseparation between said levels.

3/1960 Schawlow et al. 88--1 9/1962 Boyd et al. 881

OTHER REFERENCES Kroger: Some Aspects of the Luminescence of Solids,Elsevier Publ. Co. Inc., New York, 1948, pages 288, 290, 291, 292, 293,294, 295, 297, and 298.

TOBIAS E. LEVOW, Primary Examiner.

JULIUS GREENWALD, MAURICE A. BRINDISI,

' Examiners.

, R. D. EDMONDS, Assistant Examiner.

1. AN OPTICAL MASER COMPRISING AN ACTIVE MEDIUM CONSISTING ESSENTIALLYOF A SUBSTANTIALLY MONOCRYSTALLINE CALCIUM FLUORIDE HOST LATTICE INWHICH A PORTION OF THE CALCIUM IONS HAVE BEEN REPLACED BY NEODYMIUM IONSIN THE TRIVALENT STATE, THE PORTION OF CALCIUM IONS SO REPLACED BEING INTHE RANGE OF FROM 0.01 PERCENT TO 5 PERCENT, MEANS FOR PRODUCING APOLULATION INVERSION BETWEEN A PAIR OF OPTICALLY CONNECTED ENERGY LEVELSOF SID NEODYMIUM IONS, AND MEANS FOR STIMULATING COHERENT EMISSION ATTHE WAVELENGTH CORRESPONDING TO THE ENERGY SEPARATION OF SAID LEVELS.